Field of the Invention
The present invention relates to the field of voltage level shifting circuits, and in particular to voltage level shifting circuits for buffering signals from a circuit based on one family of digital logic to a circuit based on a second family of digital logic.
Description of Related Art
Various applications require the translation of voltages from one level to another. A prime example is the translation of digital signals between circuits based on different families of digital logic. For example, a single application may require communication between different logic family circuits for reasons of speed, power dissipation, or availability. Each family of logic devices has supply voltage requirements, current requirements, and switching voltages (threshold voltages) determined by the properties of the underlying semiconductor materials. Thus, a dependable level shifting circuit must transmit logical high and low signals between circuits having different supply and switching voltages while preserving the logical state of the signal.
The preservation of logical states during the shifting process requires the establishment of a stable switching point to distinguish high and low logical states. Conventional voltage shifter circuits often rely on the supply voltage to establish the switching point of the circuit. In addition, these circuits often employ hysterisis to sharpen the transitions between high and low logic levels. However, each of these circuit configurations imposes limitations on the operation of voltage translation circuits.
The reliance on supply voltages to establish the circuit switching point can compromise the stability of the switching point, since variations in circuit processing or temperature may change the supply voltage. In addition, the use of hysterisis to insure sharp transitions between high and low logic levels can slow the response of the voltage shifting circuit considerably, since the signal driving the circuit input must overcome the hysterisis before the output of the shifter changes state. Accordingly, there is a need for a voltage shifting circuit which combines fast response time with dependable switching characteristics.